Blow our preventers (BOPs) are well known safety apparatus for sealing off oil or gas wells during emergency situations so as to contain potentially dangerous blow-outs.
Several different types of BOP are known in the art. One of these is the ram-type BOP, which typically includes a housing having a throughbore for providing access to the well, and at least one pair of rams mounted in the housing, the rams of each pair being located on opposite sides of the throughbore. In the event of an emergency opposing rams move into, and close, the throughbore, thereby sealing off the well.
Sealing the throughbore is assisted by the provision of elastomer seal elements incorporated in the ram body. The first of these seal elements is disposed on the upward facing surface of the rams, for engaging with a seal surface machined into the ram cavity, and the second element is disposed on the inward facing vertical surface of the ram. These seal elements are positioned such that when the rams are moved to the closed condition, and the inward facing vertical surfaces are in contact, a continuous elastomer seal is formed between the ram bodies and their cavities and also between the contacting faces of the rams.
When the rams are in the closed condition and the pressure below the rams is higher than the pressure above, the rams are forced to move upwards towards the upper surface of the ram cavity. This is beneficial to sealing in two ways: firstly, the seals are pressed progressively harder against the cavity, and secondly the extrusion gap is reduced.
However, conventional BOPs have a disadvantage in that if the pressure above the rams is greater than the pressure below, these two benefits do not occur. Indeed, in such a case, the rams float downwards, away from the cavity sealing surface, simultaneously unloading the seal and increasing the extrusion gap.
This behaviour is one reason which explains why ram-type BOPs are not reliable for containing pressures from above.
Additionally, conventional BOPs are of limited use on high pressure gas wells, because, due to the space constraints within the BOP housing, it is very difficult to machine a seal seat surface in the ram cavity which is smooth enough to allow a high integrity seal to be made between the upper surface of the rams and the seal seat. This problem is accentuated if, during the closing of the rams, the elastomeric seal element on the upper surface of the rams is damaged by being dragged across the ram cavity surface.